1. Field of the Invention
This invention relates to a cold-cathode magnetron injection gun, and more particularly to a magnetron injection gun having a cold-cathode for instantaneously generating a high-power electron beam with an electron current of larger than 100 A, a duration of longer than 1 microsecond (preferably 1 to 10 microseconds), and a beam energy of larger than 100 keV, for the purpose of generating high-power pulsed electron beams or field emission electron beams. The invention intends to provide a magnetron injection gun which is suitable for application to a high-power mm microwave oscillator, a high-power X-ray generator, a high-intensity laser beam generator, a high-intensity neutron beam generator, and the like.
2. Description of the Prior Art
Principles of a magnetron injection gun using a hot-cathode were disclosed by W. E. Waters in IEEE Transaction on Electron Devices, July 1963, pages 226-234. This hot-cathode magnetron injection gun uses a conical anode and a conical cathode coaxially disposed relative to each other, and a uniform static magnetic field is applied in the axial direction of the anode and the cathode, so that electrons emitted from the cathode are prevented from reaching the anode and such electrons are extracted and used as an electron beam proceeding in the axial direction. In this magnetron injection gun, the voltage to be applied across the anode and the cathode can be low, e.g., 200 to 250 V, and a direct current beam can be extracted continuously, but the magnitude of the electron beam extracted is usually restricted to be less than several amperes because the electron beam is emitted from the hot-cathode. The reason for this restriction is that, with the hot-cathode magnetron injection gun, even if production of an electron beam of larger than several amperes is tried by increasing the static magnetic field applied from the outside and increasing the electric field at the cathode to 100 kV/cm or higher, the high electric field intensity of the electron emitting zone quickly deteriorates the function of the hot-cathode because the cathode is heated by a heater, and emission of electrons becomes impossible.
On the other hand, U.S. Pat. No. 3,344,298 of J. C. Martin et al. disclosed a diode for generating electron beams. This diode generates a pulsed beam in the form a relativistic electron beam (REB) with a power of 10.sup.9 to 10.sup.12 W, which beam is produced by applying high-voltage short-duration pulses (duration being shorter than 100 nsec) to a low-resistance planar diode having an accelerating anode disposed in the beam passage at right angles, the accelerating anode being a metallic thin film or a foil shaft. However, such electron beam diodes of the prior art have the following shortcomings.
(1) The anode foil is susceptible to breakage by the electron beam passing therethrough. PA1 (2) Collision with foil atoms tends to cause scattering of electrons. PA1 (3) Arcs generated in the diode zone tend to cause gas emission from the foil and contamination of the system. PA1 (1) A cold-cathode is used in the magnetron injection gun, instead of a hot-cathode. PA1 (2) Since the application of the same voltage as that for the hot-cathode to the cold-cathode will not cause electron emission, and an electric field intensity of 100 kV/cm or higher is applied to the cold-cathode so as to generate an electron beam having an electron current of several hundred amperes, a duration of at least one to several microseconds and a beam energy of 100 keV or more are obtained. hot-cathode PA1 (3) Due to the use of the cold-cathode instead of the hot-cathode, a high-power electron beam cannot be generated continuously, but the magnetron injection gun of the invention increases the magnitude of the current by at least ten to one hundred times and ensures a beam duration of one to several microseconds which is more than one hundred times that generated by the conventional foil-less REB diode. Thus, the magnetron injection gun of the invention generates high-power pulsed electron beams with a large electron current of several hundred amperes and the above-mentioned duration, i.e. with an electron beam density of more than 100 A/cm.sup.2, whereby high-power pulsed beams which have not been available heretofore are provided for various industrial applications.
Due to the above-mentioned shortcomings, the application of the REB diode has been limited.
M. Friedman et al. have proposed to develop a foil-less REB diode for generation of high-power annular REB without using any foil or screen as the accelerating anode which has been used in the prior art, as disclosed in The Review of Scientific Instruments, September 1970, pages 1334 and 1335 with FIG. 1.
In this proposal, a high-voltage pulse of the order of 700 kV is applied to the foil-less diode during the peak of the magnetic field with a duration of 50.times.10.sup.-9 sec., so that electrons emitted from a cathode are guided by magnetic field surrounding the cathode and formed into an annular relativistic beam extending in an axial direction. Beam generators using such foil-less diodes have shortcomings in that the magnitude of the high-voltage pulse to be applied to the cathode is too high so that the running cost becomes very high, and that the duration of emission being 50.times.10.sup.-9 sec. is too short and the field of application thereof is limited.
In the electron beam generated by the magnetron injection gun, the velocity of the individual electrons is close to the velocity of light, so that relativistic treatment is necessary. The electron beam generator of such relativistic electron beam (REB) is apparently different from conventional electron accelerators and is used in different fields.
For instance the REB can be applied to nuclear fusion, such as drivers for inertia nuclear fusion, electron guns for heating of and injection to linear plasma, formation of inverse magnetic field orientation for stabilizing plasma by REB ring beam, and the like. Besides, the REB generator can be applied to a laser generator, an X-ray generator, a neutron beam generator, a microwave generator, an ion accelerator, and the like. Thus, the REB generator is used in a wide range of fields and has been actively studied in recent years.